[Update] mos_mu tool must check availability of mos-update repo in cluster and add it if necessary.

[Ilya Bumarskov]

Fuel 9.0 (deployed via fuel-qa with FORCE_DISABLE_UPDATES=True)

Detailed bug description:
Preparation playbook (ansible-playbook playbooks/mos9_prepare.yml -e '{"env_id":<env_id>}') returns error if mos-updates repository wasn't added in cluster.

Steps to reproduce:
   - Deploy 9.0 env withfollowing nodes:
         * Controller
         * Compute, cinder
   - Download mos-mu tool on master (git clone https://github.com/aepifanov/mos_mu.git)
   - Make a perform a preparation playbook (ansible-playbook playbooks/mos9_prepare.yml -e '{"env_id":<env_id>}')

Observed behavior:
Task [Fix apt-preferences] is failed because nodes in cluster don't contain the mos-update repo.
http://paste.openstack.org/show/588232/

[Andrey Epifanov (aepifanov)]

Merged:
https://github.com/aepifanov/mos_mu/commit/928717ab0debeeadeeee0b1d35f9275b5cf86dcc